Implantable systems for delivering high-energy shocks to defibrillate the heart conventionally use single or multiple simultaneous or sequential electrode vectors to deliver a defibrillation waveform. A single electrode vector, for example, between an electrode located in the right ventricle and an electrode placed outside the right ventricle, often results in undesirably high energy levels being required in order to effectively defibrillate the heart (defibrillation threshold). In delivering a defibrillation shock, it is desirable to deliver the energy in a vector substantially parallel to a large mass of the cardiac myocytes in order to simultaneously depolarize the myocytes and “reset” the timing of myocyte firing, thereby restoring normal sinus rhythm. This shock directionality is approximated through the positioning of defibrillation electrodes relative to the heart. However, because the cardiac structure is complex, a defibrillation pathway selected between two defibrillation coil electrodes, between a defibrillation coil electrode and the implantable device housing used as a “CAN” electrode, or between a defibrillation coil electrode and a subcutaneous patch electrode, may be substantially parallel to a limited cell population.
In an effort to reduce the amount of energy required to effect defibrillation, numerous suggestions have been made with regard to multiple electrode systems. For example, sequential pulse multiple electrodes systems are generally disclosed in U.S. Pat. No. 4,708,145 issued to Tacker et al., U.S. Pat. No. 4,727,877 issued to Kallok et al., U.S. Pat. No. 4,932,407 issued to Williams et al., and U.S. Pat. No. 5,163,427 issued to Keimel.
An alternative approach to multiple electrode sequential pulse defibrillation is disclosed in U.S. Pat. No. 4,641,656 to Smits and also in the above-cited Williams patent. An alternative multiple electrode, simultaneous pulse system is disclosed in U.S. Pat. No. 4,953,551, issued to Mehra et al., employing right ventricular, superior vena cava and subcutaneous patch electrodes.
Pulse waveforms delivered either simultaneously or sequentially to multiple electrode systems may be monophasic (either of positive or negative polarity), biphasic (having both a negative-going and positive-going pulse), or multiphasic (having two or more polarity reversals). Such waveforms thus include one or more pulses of negative and/or positive polarity that are typically truncated exponential pulses. While the term “multiphasic” is used to refer to a pulse waveform having two or more polarity reversals, the waveform may be described as a “multiple pulse” waveform that includes both positive and negative pulses with intervening pulse delays. These monophasic, biphasic, and multiphasic pulse waveforms are achieved by controlling the discharge of a capacitor or bank of capacitors during shock delivery.
Simultaneous multiple electrode defibrillation configurations provide a defibrillation pathway along more than one vector simultaneously producing a net vector field. However, in multiple electrode configurations, each pathway or vector will have an associated resistance. When multiple pathways are used simultaneously, a current divider effect is created. The path with the least resistance will receive the majority of the defibrillation shock current.
In sequential multiple electrode configurations, a defibrillation waveform is typically delivered along two current pathways sequentially such that one defibrillation vector is produced followed by a second defibrillation vector. The directionality of the sequential vectors is generally limited to two distinct vectors determined by the location of the electrodes used to deliver each pulse. Even when using multiple electrode configurations, a relatively high-energy shock is still required in order to successfully defibrillate the heart.
Reducing device size to an acceptable implantable size was a major obstacle in realizing the first implantable defibrillation devices. Large battery and capacitor requirements for delivering high-energy shock pulses required early devices to be relatively large. Using truncated biphasic exponential waveforms for internal cardiac defibrillation via transvenously positioned electrodes has allowed defibrillation thresholds to be reduced to the point that device size is acceptable for pectoral implant. However, relatively high energy requirements still continue to limit device longevity and size reduction, both of which continue to be motivating factors to improve implantable defibrillation systems by reducing the defibrillation thresholds required to successfully defibrillate the heart. Reduced defibrillation energy may also reduce sensitivity to lead placement and differences in cardiac anatomy and thereby reduce the number of patients in which unacceptable defibrillation thresholds are encountered.
As discussed previously, reduction in defibrillation thresholds may be achievable if a greater number of the cardiac myocytes are parallel to the defibrillation vector field. One approach to addressing this need could be to increase the number of electrodes to allow delivery of simultaneous or sequential defibrillation pulses along a greater number of vectors. Placement of additional electrodes however, adds size, cost, and complexity to the implanted system and would make implantation of the system an arduous task.
There remains a need, therefore, for an improved system and method for defibrillating the heart using a multi-directional defibrillation vector field for achieving successful defibrillation at lower shock energies and that allows a reduction in implantable device size and/or extension of the useful life of the implanted device. By reducing the defibrillation energy required, the number of patients in which acceptable defibrillation thresholds are unachievable may also be reduced.